C Gattman wrote:

At a towered airport, walking onto an active taxiway is considered a
runway incursion.


No it isn't. A runway incursion is "any occurrence at an aerodrome
involving the incorrect presence of an aircraft, vehicle or person on the
protected area of a surface designated for the landing and take off of
aircraft."


At the same airport, taxiing onto an active taxiway without clearance
is a runway incursion.


Only if it's been designated for the landing and take off of aircraft.